Change the "major dimension" that Orthographic Scale affects?
A bit confusingly worded question, but what I'm gathering is you want to know how to exclusively scale along the X, Y, and Z axes? (X and Y are horizontal, Z is vertical). All you have to do is after pressing S to scale, simply press X, Y, or Z. This will restrict your scale to whatever dimension you chose. Also, if you press Shift+X, Y, or Z after pressing S, this will restrict scaling to every axis except the axis you selected. For example, S then Shift+Z will make your scaling only scale along the X and Y axes.